The project is concerned primarily with ultrastructural studies of membrane specializations in the electrically excitable regions of nerve fibers. Particulate components of the axolemma at the node of Ranvier and junctional specializations in the paranodal region have been demonstrated in normal nerve fibers, and abnormalities in both regions have been demonstrated in a number of mutant animals that have defective myelin. During the continuation period of this grant ultrastructural studies including freeze-fracture studies will be continued on other myelin-deficient mutants including the mouse known as shiverer. These studies are designed to provide additional evidence bearing on the extent to which the myelin sheath determines the structural and physiological properties of the axolemma.